Vigorous studies are in progress concerning large-capacity optical communication networks by wavelength-division multiplexed optical signals as an infrastructure supporting the prospective information-oriented society. It has been recognized through the studies that an epoch-making improvement in performance of networks will be possible if signal wavelengths can be converted at nodes of networks.
With this recognition, major research organizations through the world are trying to present practical wavelength converting techniques. Basically, they propose wavelength converters using mutual gain saturation, four-photon mixing or mutual phase modulation of semiconductor laser amplifiers, or using four-photon mixing of optical fibers. FIGS. 11 through 14 are block diagrams showing general constructions of prior devices.
The prior device shown in FIG. 11 uses mutual gain saturation characteristics of a semiconductor laser amplifier. Fed to one end of a semiconductor laser amplifier 110 are intensity-modulated signal light having the wavelength .lambda.1 and continuous (CW) light having the wavelength .lambda.2. Due to mutual gain saturation characteristics of the semiconductor laser amplifier 110, signal light of wavelength .lambda.2 output from the other end of the semiconductor laser amplifier 110 results in intensity-modulated data in which data is inverted in the signal light of wavelength .lambda.1.
The prior device shown in FIG. 12 arranges two semiconductor laser amplifiers 112, 114 in form of a Mach-Zehnder interferometer using couplers 116, 118 and 120, and utilizes mutual phase modulation characteristics of the semiconductor laser amplifiers 112 and 114. When a data optical signal of wavelength .lambda.1 from the coupler 116 and continuous light of wavelength .lambda.2 from the coupler 118 are fed to the Mach-Zehnder interferometer, data optical signal output of wavelength .lambda.2, which has been intensity-modulated similarly to the data optical signal of wavelength .lambda.1, is obtained from coupler 120.
The prior device shown in FIG. 13 uses four-photon mixing of a semiconductor laser amplifier. When a data optical signal (wavelength .lambda.1) and pump light are fed to a semiconductor laser amplifier 122, its output light results in containing satellite light and data optical signal light of wavelength .lambda.2 in addition to the data optical signal (wavelength .lambda.1) and the pump light, due to four-photon mixing. The data optical signal light of wavelength .lambda.2 is a converted light.
The example of FIG. 14 uses a non-linearity of an optical fiber 124 (for example, typical quartz optical fiber) in lieu of the semiconductor laser amplifier 122 of FIG. 12.
Among them, the device shown in FIG. 11 is simplest in construction. However, its extinction ratio of converted light is insufficient and not suitable for multi-stage wavelength conversion.
The device shown in FIG. 12 needs a complicated structure arranging two semiconductor laser amplifiers 112, 114 in form of a Mach-Zehnder interferometer. Moreover, since even a small optical power results in phase modulation of 180 degrees, it invites the problem that small fluctuations in power of the original signal light delicately affect the converted light.
Also the systems shown in FIGS. 13 and 14 using four-photon mixing have following problems. Namely, the arrangement using the semiconductor laser 122 (FIG. 13) has a narrow wavelength band for conversion, and invites a negligible deterioration in S/N ratio by spontaneous emission. The arrangement using the optical fiber 124 (FIG. 14) needs a long optical fiber 124 not less than 1 km, and inevitably causes a large dimension of the device. Moreover, although four-photon mixing theoretically requires that the plane of polarization of the original signal light coincides with that of the pump light, the plane of polarization of signal light travelling through an optical fiber fluctuates on time, and therefore it needs means for removing the fluctuations.